Chapter 6: Conclusion

Outline

This chapter brings together the conclusions from each chapter
and describes the current status of the COAST infrared project.
Some suggestions for improvements and future directions are included.

Status

The COAST project has proved that the concept of optical interferometry
with arrays of small telescopes is practical and technically possible.
The telescope has recently produced its first map, an image of
the binary star Aur (Cappella) with a resolution of 20 milli-arcsecond.

At the end of this project the COAST telescope is almost complete.
The infrared system is capable of making astronomical observations
in the 'J' and 'H' bands with modifications for the 'K' band possible.
The telescope is operational and able to measure fringes on astronomical
targets routinely. Although the infrared system is not aligned
perfectly, the experience with the visible system suggests that
only a little extra effort is required.

The camera built for COAST has performed well. The aim of building
a system based on visible CCD camera technology and the techniques
has been proved. In particular this route has produced an infrared
camera with exceptionally good read noise.

Improvements to COAST

The COAST infrared system can potentially produce a great deal
of astronomical data. Although in its current state it achieves
most of the design aims there are a number of improvements and
additions which can be made. These are listed below in decreasing
order of practicality.

Enhanced Optical Coatings

Now that the design of the infrared system has been proved to
work, a number of components can be replaced with items optimised
for the infrared. The most obvious of these are the windows at
the entrance to the optics lab which have an anti-reflection coating
optimised for visible wavelengths and so absorb a large proportion
of the infrared light. In addition to this the camera lens and
dewar window can be given anti-reflection coatings which will
reduce the flux losses from these components by around 25%. These
anti-reflection coatings have already been designed and tested
on the beam-splitter compensating plates. The anti-reflection
coatings will almost eliminate the flux losses in the transmission
components and so this would double the flux reaching the detector.

Fringe tracking

At short baselines the position of the path compensation trolleys
can be easily calculated from the co-ordinates of the star and
the geometry of the baseline. At longer baselines the extra atmospheric
path introduces an uncertainty in the position of the fringe envelope
which is larger than the coherence length. In this case the fringe
visibility would drop as the light from different telescopes moved
out of phase.

To obtain high visibility fringes continually, it is necessary
to track the movement of the fringe envelope and correct for any
extra atmospheric path. This would be done the controlling system
continually receiving measurements of the fringe visibility in
real-time and responding to a drop in visibility by moving the
path compensation trolleys. Unfortunately it is not possible from
just a change in visibility to know in which direction to apply
the correction. However if the system observed simultaneously
at several wavelengths then by monitoring if the visibility dropped
more quickly at shorter or longer wavelengths the servo loop can
be closed. The possibility and advantages of multi-wavelength
operation is described below.

Multi-wavelength observing

It may be possible to extend the infrared system to simultaneously
observe at a number of different wavelengths. All the components
in the telescope and path compensation system are achromatic.
The beam-splitters were also designed to operate at all infrared
wavelengths. The output of the correlator is four parallel beams
containing all the light from the telescopes, modulated into a
fringe pattern. It is only the filter in the camera which selects
which of the wavelengths in these beams actually reaches the detector.

In the visible system a multi-wavelength approach has been suggested
largely in an attempt to improve the light gathering efficiency.
Since the visible system observes in narrow bands, typically 10
nm, a large fraction of the light is wasted. If the outputs of
the beam combiner were dispersed into a spectrum then each 10
nm width of this spectrum could be imaged onto a separate detector.
In the infrared case it would be simplest to disperse each beam
into only two pixels giving two wavelength channels in each infrared
band. This could be achieved with a thin prism placed after the
camera lens and would require no changes to the dewar. There is
a reduction in the signal to noise since only half the light is
detected by each pixel, but the read noise remains the same. However
for bright objects this would be acceptable. The case of two wavelength
channels in the same infrared band is easily possible. An alternative
scheme could be to disperse the entire infrared region so that
the J and H bands fell into single pixels with the atmospheric
water band between them. This would remove the need for the infrared
band selecting filters, but a cooled short-pass filter would be
needed to prevent the thermal background at longer wavelengths
from reaching the detector.

K band operation

The current prototype camera cannot be used for observations with
COAST at the longest near infrared wavelength , K band = 2.2m.
The simple dewar arrangement allows too much of the thermal background
emission, from the instrument and building, to reach the detector.
To avoid this background signal each pixel must be allowed to
only 'see' the beam of light from the correlator or cold components
inside the dewar. The simplest solution to this problem is to
extend the front of the dewar so that there is a cold field stop
at the position of the camera lens. Although the majority of the
detector elements would still receive a signal from the background,
the four pixels used by the instrument could only receive light
from the output beams of the beam combiner.

The original reason for considering a 'K' band system was the
expected improvement in the observations of fainter objects. The
improved atmospheric conditions at longer wavelengths should allow
larger telescopes and wider bandwidths to be used. However in
COAST the fixed size of the telescope means that there is little
improvement in limiting magnitude when moving to the longer wavelengths.
Although a wider observing bandwidth should be possible from purely
atmospheric considerations, the thermal background emission at
longer wavelengths means that a narrower filter must be used.

Larger mirrors

As described in chapter 1, a major reason for working in the infrared
is the improved atmospheric conditions. The most significant effect
of this is the ability to use much larger aperture telescopes
while achieving the same level of wavefront perturbations. Table
4.2 in chapter 4 shows the size of possible telescope for each
band if apertures 2.8 r0 were used. Observing in the
K band would allow telescopes of 1.5m aperture to be used, giving
14 times the collecting area of the 0.4m telescopes used currently
at COAST.

The COAST telescope primary mirrors could be replaced with larger
apertures, since they are fixed and do not need to be supported
under a varying load. The only difficulty with increasing the
mirror size is the cost. The siderostat mirrors are more difficult.
It would be probably be impossible to use large conventional glass
mirrors with the current mount design. New technologies being
developed for space applications such as composite or thin metal
mirrors could offer a solution.

Better site

Although COAST is designed to avoid the atmospheric seeing which
limits the resolution of conventional telescopes it is not totally
immune to atmospheric affects. The size of the telescope apertures
and the length of the exposures are directly linked to the stability
of the atmosphere. By moving to a good astronomical site COAST
will be able to make use of larger mirrors, particularly at shorter
visible wavelengths, and so will be able to observe fainter objects.
Unfortunately most good astronomical sites are on mountain tops
which lack the large flat areas needed to build an interferometer.